There is known from Schleupen et al U.S. Pat. No. 4,918,389, the disclosure of which is hereby incorporated by reference to which published European Patent Application 0 344 349 corresponds, a vehicular engine ignition system in which the monitoring is performed by monitoring the ignition spark duration through a connection to the primary side of an ignition coil. This kind of monitoring has the disadvantage that under certain operating conditions measuring the spark duration is not a reliable monitoring method. In particular where the high voltage is distributed by a rotary distributor, the sparks do not burn out at high engine speeds but are extinguished by the switching in again of the energizing circuit of the primary of the ignition coil.
Known monitoring procedures for ignition systems are, for example, the monitoring of catalyst temperature and the measurement of engine noise, of the lambda sonde signal and of the duration of the ignition spark.
These known methods do not provide results equally good in all ranges of operation of the engine, as has already been mentioned with reference to the measurement of the spark duration. External influences can also falsify the results, as for example in the case of engine noise measurement, which is impaired by road and power train irregularities (potholes and engine jerks).